1. Field of the Invention
The invention relates to a rail having high resistance to wear in its head and high resistance to rupture in its foot.
2. Discussion of the Prior Art
Rails for rail vehicles should on the one hand have high resistance to wear in the head and on the other hand, because of the flexural tensile stress in the track, high resistance to rupture in the foot. As resistance to wear increases and resistance to rupture decreases with increasing strength of the rails, it has not hitherto been possible to improve both properties simultaneously in one material composition. A solution was seen in the so-called two-component rail, which through composite casting was composed of a high-strength material with high wear resistance in the rail head and of a soft material having good toughness properties in the web and foot of the rail. Because of the low strength in the rail web and foot, such rails, however, are not suitable for heavy stresses, since they undergo plastic deformation under heavy axle loads (22 tons). Moreover, in the region of the transition of the materials metallurgical disturbances cannot be avoided with adequate certainty. These may lead to fatigue fractures. Composite casting rails have, therefore, not been used for a long time.
In rails, which in their air-cooled naturally hard state have a pearlitic structure, other solutions seek either to increase strength in the rail head by subsequent heat treatment (as, for example, the German Journal "Stahl und Eisen" 90, 1970, No. 17, pp. 922-928) or to improve toughness in the rail foot by heat-treating the pearlitic structure of the rails in their state of natural hardness (Austrian Patent Specification No. 259,610). An optimum solution is also not achieved in this manner, since even with a heat-treated pearlitic structure in the rail foot only a light improvement of resistance to rupture can be achieved.
The resistance of rails to brittle fracture has hitherto been judged solely on the basis of the insensitivity of the rail steel to brittle fracture. Characteristic values in this respect are determined by tensile tests and rail impact tests. They are a measure of the deformability of steel before rupture. These tests are carried out as part of the acceptance trials for the rails. They have proved satisfactory in the judging of the rupture resistance of rails. In individual cases further information is obtained from notched bar impact bending tests in dependence on the test temperature. However, all these test methods permit only comparative grading of steels, while quantitative application of test results to the behavior of the component, that is to say in this particular case to the rail in the track, is not possible.
In order to be able to assess quantitatively the rupture resistance of rails, in recent times use has been made of crack resistance, determined as a characteristic value of the material in mechanical fracture investigations, to judge rupture behavior. Crack resistance is thereby determined in accordance with ASTM Standard E 399-74.
The crack resistance test is described in detail in DE-Z "Tech. Mitt. Krupp Werksberichte", vol. 39 (1981), No. 1, pp. 33-42.
From this publication, it can further be seen that rail steels having strengths above 900 N/mm.sup.2 according to the prior art, for example, in accordance with UIC-Kodex 860 V, in the hard-rolled state or in the heat-treated pearlitic structure state usually have crack resistance values of 1000 to 2000 N/mm.sup.3/2. In the hard-rolled state the tensile strength of standard rails is above 900 N/mm.sup.2 and the yield point is above 450 N/mm.sup.2. With heat treatment followed by cooling to a fine pearlitic structure in the head or over the entire cross-section of the rail, it is true that these values can be raised to 1100 N/mm.sup.2 for tensile strength and to 600 N/mm.sup.2 for the yield point, but crack resistance is scarcely changed. In general, it can be said that rail steels, which, based on analysis, have higher strength values, show poorer crack resistance values in the lower scatter range. This means that these rails show better wear behavior in the track, but have an increased tendency towards brittle fracture, particularly at high axle loads above 22 tons.
Taking this prior art as starting point, it is an object of the present invention to provide rails having an optimum combination of high strength in the head and high crack resistance in the foot, and which, in the track, and even with high axle loads above 22 tons, show good wear resistance in the head and such high resistance to rupture in the foot that plastic deformations and brittle fractures are avoided.